Description: Both, the analysis of polar climate change on the basis of observations and the validation of weather and climate prediction in polar regions are challenging since only few observations are available. In the inner arctic regions in-situ observations are available only from buoys, ship cruises and aircraft campaigns with large temporal differences and spatial separations. In the present contribution we compare near-surface meteorological observations and rawinsonde soundings from Arctic cruises with the German icebreaker RV Polarstern during August 1996, 2001, and 2007 with each other and with ERA-Interim reanalyses. Although the used observations are usually applied in the reanalysis, they differ considerably from ERA data. ERA overestimates the relative humidity and temperature in the atmospheric boundary layer and the base height of the capping inversion. Warm biases of ERA near-surface temperatures amount up to 2K. The melting point of snow is the most frequent near-surface temperature in ERA, while the observed value is the sea water freezing temperature. While this points to general drawbacks in the models, it shows also that the quantification of trends based on reanalyses is problematic especially when only one reanalysis is considered.

Description: Climate change is especially pronounced over the Arctic Ocean, where the atmosphere warmed twice as fast as in lower latitudes in the last few decades. This warming is associated with a rapid decline of the Arctic sea ice cover. For future predictions of changes in the Arctic climate system, profound knowledge of all processes influencing the surface energy budget in polar regions is essential. The focus of this thesis lies on improving our current understanding of convective processes and the related turbulent fluxes in the polar atmospheric boundary layer (ABL) over both the sea ice covered regions and over the open ocean at the sea ice edge. A major part of the analysis is based on aircraft measurements from the campaign STABLE, which was carried out over the pack ice in the northern Fram Strait in March 2013. These results are supplemented by modeling studies using a simple boxmodel and a one-dimensional mesoscale model. For the first time, comprehensive aircraft measurements over leads were conducted during the campaign STABLE. They are used to study the formation of convective plumes over leads and their impact on the polar ABL. It is found that the conditions over four wide leads are highly variable with respect to turbulent fluxes, as well as to the mean variables temperature, humidity, and wind. In one of the cases large entrainment fluxes exceeding 30 % of the surface fluxes are observed. The convective plumes over leads have a large influence on the vertical profiles of sensible heat and momentum fluxes, which are non-linear downstream of the leads with a distinct flux maximum in the core of the convective plumes. For the first time, it it shown based on measurements that the plume also affects the wind field by diminishing low level jets in the region influenced by the plume. In addition to the small scale impact of individual leads the regional impact of lead ensembles is studied using long transect flights. The analysis shows that near-surface atmospheric temperatures are clearly related to the ice concentration in the considered region. The impact of a heterogeneous sea ice cover and of the related surface temperature changes on atmospheric temperatures is also analysed using a Lagrangian box model. The model uses reanalysis winds as well as sea ice concentration and surface temperature from satellites as input data. The box model is used to calculate the evolution of the near-surface air temperature along backward-trajectories, which are then compared to measured temperatures at three different Arctic sites. The results suggest that a large amount of the observed air temperature variability can be attributed to heterogeneous surface temperatures and that the characteristic length of the upstream region influencing air temperatures at a specific location is 200 km. Convection during cold air outbreaks at the sea ice edge has a much stronger impact on the polar ABL than convective plumes over leads. Dropsonde measurement of four cold air outbreaks during STABLE are used to analyse the downstream development of meteorological variables and the ABL growth. Two of the considered cases are influenced by the size of the Whaler's Bay polynya north of Svalbard, which was unusually large in the three winters from 2012 to 2014 compared to the previous 20 years. The analysis of the dropsonde measurements shows that the unusual ice conditions lead to strong atmospheric convection in a region north of Svalbard that was typically ice-covered in the last decades. This leads to extreme convective ABL heights and modifies local temperature conditions considerably. Convective processes in the ABL have to be parametrised in climate models. Therefore, in addition to the measurements, the performance of three different sensible heat flux parametrisations is tested in a 1D mesoscale model and results are compared to those of a large eddy simulation model (LES). Both the considered counter-gradient and eddy-diffusivity mass-flux (EDMF) approach reproduce the shape of the temperature profile of the LES better than a classical mixing length approach. A sensitivity analysis shows that the EMDF approach is the least sensitive to changes of the vertical grid spacing, which can be attributed to the derivation of the ABL height using a diagnostic equation of the updraft velocity. The sensitivity of the counter-gradient closure to the grid spacing can be significantly reduced when the updraft velocity equation of the EDMF approach is included and used to derive the ABL height.

Description: The Weather Research and Forecasting (WRF) Model ARW V3.3 was applied to simulate the atmospheric boundary layer (ABL) modification over the Ronne Polynya in March 2010. The model results were validated against airborne observations from two research aircrafts. These observations were made during the campaign Joint Airborne Study of the Peninsula Region (JASPER) by the British Antarctic Survey and the Alfred Wegener Institute. Throughout the 36-h-long simulation period on 1-2 March, there was a south to south-westerly mean flow, coming from the ice shelf towards the polynya, which was approximately 10 km wide. Three model experiments were made, all of them receiving boundary conditions from the ECMWF operational analyses. The only difference between the model runs was in the sea ice concentration. The first model run was conducted with the sea ice concentration of the ECMWF operational analysis, which was available in a resolution of 16 km. For the second model run, a sea ice dataset with a much finer resolution was used, the AMSR-E dataset refined by the ARTIST Sea Ice algorithm, in a resolution of 6.25 km. In the third model run the polynya was manually replaced by a compact sea ice cover. Thereby the sensitivity of the ABL modification over the polynya to the sea ice data was analyzed. It is shown that the finer spatial resolution of the AMSR-E sea ice data allows a better simulation of the local polynya effects on the ABL, especially an increase in air temperature, humidity, and near-surface wind speeds. The validation against the observations revealed a cold temperature bias in the model within the ABL under stable to near-neutral conditions, and above the ABL regardless of stratification. The surface fluxes of heat and moisture and the ABL height were investigated along three cross-sections across the polynya. Additionally, an ice breeze from the sea ice surrounding the polynya towards the center of the polynya, and back to the sea ice at higher altitudes, was found in the WRF simulation with the AMSR-E sea ice dataset.